1. Field of the Invention
The present invention relates to a video signal selection circuit, and more particularly, to a video signal selection circuit that can recognize automatically the connection state of a plurality of video jacks and select only those modes which correspond to connected jacks when an audio/video tape apparatus is switched via conversion actuator from one mode to another.
2. Description of the Background Art
Audio/video tape apparatuses, especially televisions (TVs), generally have a plurality of jack ports, whereby the number of ports varies with the sophistication of the particular model. Generally, jacks can be provided for any of the following signal sources: radio frequency (RF), video 1, video 2, video 3 and super video (S-VHS).
FIG. 1 is a block diagram depicting a conventional sensing circuit for a video jack block. This circuit includes a key input portion 101 having ten number keys and a mode conversion key (e.g. TV/video conversion key), a micro-processor 102 and a switching portion 103. The micro-processor 102 is programmed to control a TV and regulate a plurality of video modes so that they are convened sequentially by actuating the mode conversion key of the key input portion 101. The switching portion 103 selects an appropriate source of the TV by switching a corresponding jack port and an output terminal of the jack block 104 according to the control of the micro-processor 102.
The kinds of video jacks provided vary according to the model of the TV, i.e. the higher the grade of the TV, usually, the larger the number of video jacks. Hence, the video modes controlled by the mode conversion key are set up by means of option diodes, as shown in the following Table
TABLE 1 ______________________________________ Diode 1 Diode 2 Video Loop ______________________________________ 0 0 TV .fwdarw. VIDEO 0 1 TV .fwdarw. VIDEO 1 .fwdarw. VIDEO 2 1 0 TV .fwdarw. VIDEO 1 .fwdarw. VIDEO 2 .fwdarw. S-VHS 1 1 Only TV ______________________________________
For example, if the state of diodes 1 and 2 is set to 10, then there are three jack ports provided in the video device. At this point, assume that only the jack port corresponding to video 1 is connected with a jack, and that jack ports for video 2 and super video are not connected to jacks. If the present mode is the video 1 mode and the mode conversion key of the key input portion 101 is actuated, the micro-processor 102 simply proceeds to the next port in the sequence and controls the switching portion 103 to select the video 2 mode. If the mode conversion key is pressed again, the super video mode is selected.
Thus, if the current mode to be selected is the TV mode, then the mode conversion key must be pressed three times from the video 1 mode. Since the jack ports for video 2 and super video are not connected to a jack, there is no signal input at these ports, and a black or blue screen is displayed.
In the conventional sensing circuit described above and depicted in FIG. 1, if the mode conversion key is pressed, even the modes not used are selected at their point in the sequence. This results in inconvenience to the user, who must press the mode conversion key several times to select the mode he or she wants.
In a related technological development, Japanese Unexamined Publication No. 92-235478 (published on Aug. 24, 1992) and entitled "External Input Selection Circuit", discloses an art that yields substantial reductions in cost by means of circuitry capable of selecting from among a plurality of external inputs without the need for a plurality of synchronous signal detection circuits.
If a user employs this circuitry to convert from a television mode to an external input mode conversion, the conversion circuitry commences a sweep of the tuning voltage, and the various external inputs are sequentially selected according to the change of this tuning voltage.
If a vertical synchronous signal is detected from one of the selected external inputs, the sweep of the tuning voltage terminates, such that selection of that external input is maintained. Therefore, vertical synchronization signal detection circuitry need detect only those external inputs selected by the conversion circuit, and there is no need to install a vertical synchronous signal detecting circuit for each of the plurality of external inputs, which results in reduction in cost.
However, it is possible that even an external input that is not connected to an external source may be selected at the time of selecting external inputs.